Participants in this research have Parkinson's disease and a fluctuating response to levodopa. The purpose of this study is to determine if apomorphine, an investigational (experimental) dopamine agonist (a drug that acts in a manner similar to dopamine in the brain) can completely substitute for levodopa for 3 days. Both domperidone (an anti-nauseant drug) which will be used in this study and apomorphine are experimental drugs.